Expression of cell surface associated properties during chemical carcinogenesis is being investigated in animal and human in vitro model systems in order to identify cellular characteristics useful in isolating preneoplastic cell populations and for use in preventing neoplastic disease through interventive treatment early in carcinogenesis. Fibrinolytic activity, lymphotoxin sensitivity, induction of a sustained natural tuberculin-like response, and threshold tumorigenic inoculum in adult syngenic animals have been found to be quantitatively related properties. A system of acid stable, autocrine-like protease inhibitory molecules functioning as regulators of local tissue protease activity has been described. New surface antigens associated with preneoplastic development expressed during carcinogenesis in the hamster are also found on a few neoplastic cell lines and on 10-day gestation hamster embryo cells. Monoclonal antibodies produced to 10-day embryo cells are being characterized with regard to the nature of the antigens recognized and their expression during carcinogenesis. Studies include protease stimulation of lymkphotoxin production, oncofetal antigen expression during carcinogenesis and relationship of cell surface properties to the developmental state of the cells.